1. Field of the Invention
The present invention is directed to pattern generation in photolithography systems.
2. Related Art
In maskless lithography systems, image patterns are generated by a micro-array of mirrors at an object plane of a target wafer. These mirrors can be tilted in a controlled manner to produce grey-scaling. On the wafer scale, these mirrors (pixels) can be demagnified to as little as a few tens of nanometers. In spite of this, because of the increasingly small and diverse patterns that are being targeted in deep ultraviolet (DUV) lithography, the critical dimension (CD) or pitch of these patterns is not necessarily a multiple of the pixel size.
CD and pitch are not necessarily big limitations for isolated patterns. However, finding an adequate pixel layout over a continuous range of pitches for grouped patterns can be complicated and requires a systematic approach for not only generating images with the appropriate pitch and periodicity, but also for generating images that will meet standard image quality requirements.
Traditional photolithographic images are produced using a glass or fused silica mask that is encoded with a particular image. An underside of the mask is then coded with chrome or other similar material. Focused light is then passed through the mask to project an image onto a recessed substrate where the image can be captured. Light passes through the transmissive portion of the mask to form light portions of the image while the chrome material on the underside of the mask acts to absorb light to form dark portions of the image. Each mask is configured to produce only a single image.
Maskless lithography provides many benefits over lithography using conventional reticles. One of the greatest benefits of a maskless lithographic system is the ability to use a single programmable mask to produce multiple lithographic images. As known in the art, maskless reticles include an array of thousands of micro-mirrors. The array of mirrors serves as a programmable array of light modulators, where a deflected mirror corresponds to a dark portion of a desired pattern and an undeflected mirror corresponds to a bright portion of a pattern with gray levels for intermediate states. An illumination source is projected toward the micro-mirror device to produce an image on a substrate. A spatial light modulator (SLM) and a digital micro-mirror device (DMD) are examples of maskless reticle systems currently used in photolithography.
Although mask based reticles can only be used to produce a single image, the image produced by the mask based reticles is typically of a higher quality than images produced by SLM's. One of the issues contributing to some degradation in images produced by maskless reticles is that these images face some limitations due to size and other restrictions associated with the dimensions and characteristics of the mirrors.
For example, in a maskless lithography system the mirrors can be tilted in a controlled manner to produce a grey-scaling. On the wafer scale, these mirrors or pixels can be demagnified to as little as a few tens of nanometers. However, due to the size and other limitations of the pixels and because of the increasingly small and diverse patterns that are being targeted in deep ultraviolet (DUV) lithography, the CD or pitch of these patterns might not be a multiple of the pixel size, as noted above.
What is needed therefore is a method and system to accommodate the creation of a wider variety of patterns for maskless lithography systems. More specifically, what is needed is a technique for maskless lithography using mirror arrays of a fixed size to print nested lines and contact hole at a variety of pitches.